Threatened Species Unit, Western
The State of New South Wales, Department of Environment and Conservation, 2004
10 Management issues
- 10.1 Threats and reasons for decline
- 10.2 Social and economic consequences
- 10.3 Biodiversity benefits
- Grazing pressure
- Weed invasion
- Physical disturbance
- Collection of plants
Historically, clearing has had a massive impact on the distribution of C. arenaria as inferred by the historic and current distribution, and the habitat it occupies. Fitzgeralds original identification of the species as "growing on sand-hills among pines" is informative. This description accords broadly with formerly widespread and abundant vegetation types on sandier soils of rises in the Riverina. The habitat is likely to have included woodlands dominated by Grey box (E. microcarpa), Yellow box (E. melliodora) and White Cypress Pine (C. glaucophylla).
Over 80 percent of vegetation fitting this description has been cleared in the area between Ardlethan, Corowa and Deniliquin (White, M.D., Muir, A. and Webster R. in prep.) and most of that remaining has been modified, often substantially by sheep and rabbit grazing, forestry practices and other factors. The population on private property near Urana occurs adjacent to cropped pasture on the eastern and western sides. Presumably prior to clearing the population would have been more extensive.
Grazing has the capacity to eliminate orchids or severely reduce their reproduction success. Leaves and scapes (flower stems) are palatable and are often observed to have been grazed, in situations accessible to native and introduced vertebrate herbivores. Sheep and goat dung was found in all three quadrats placed among the C. arenaria population in Buckingbong State Forest in 2000. Heavy grazing may reduce the viability of populations by limiting the rate of reproduction and lowering the rate of resource acquisition. Adult mortality is likely to be increased in these circumstances, and coupled with insufficient recruitment, population numbers will fall. There may also be predation of the tubers by various animals, such as White-winged Choughs, rabbits or pigs.
Rabbits (among other factors) may have been responsible for eliminating the species from sand-hills on the Riverine Plain. For example, on Tupra in 1890 almost one million were killed (Semple 1990). The animals can more readily burrow into the lighter soils of the sand-hills than the heavier clay soils of the surrounding plain. Given the rabbit plagues that occurred prior to the release of myxomatosis in the 1950s, it is not surprising that the species is extinct in these areas.
With domestic stock, particularly cattle, there is the added concern of pugging damage in wet conditions. Further, grazing may adversely effect plants the pollinator may rely on, or soils in which the female wasps construct nests. The level of grazing a C. arenaria population can sustain without being adversely affected is not known.
At Urana C. arenaria occurs within a fenced area that has not been stocked for some years, and in an adjacent unimproved pasture under a set stocking regime using sheep. In the fenced area the plants are scattered among regrowth White Cypress Pine. In the pasture most plants were found growing adjacent to, or within clumps of Lomandra effusa, an unpalatable grass-like plant with tough spiky leaves. The implication is that sheep grazing has eliminated or suppressed growth of C. arenaria in the open areas between the clumps of Lomandra effusa.
There are a number of weeds that are potentially impacting on populations of C. arenaria. In some situations there is greater than 80% projected foliage cover of weeds, predominantly exotic annual grasses (eg. An asterisk denotes exotic speciesBromus diandrus, Great Brome and *Vulpia spp., Fescues). With such a significant proportion of the understorey composed of exotics, some reduction in resources (light, moisture) available to C. arenaria is likely. The absolute magnitude of impact on C. arenaria is not known but potential for harm is believed to be major, particularly in the longer term. Near Wahgunyah State Forest a C. callitrophila population (also an endangered Riverina endemic) is being destroyed by the invading exotic Ehrharta calycina (Perennial Veldt-grass).
Of the four populations that have been surveyed systematically, all have hybrids present. Of greatest concern is the population on the TSR roadside, where hybrids outnumber C. arenaria. In Buckingbong State Forest there are double the number of C. arenaria as hybrids. At Urana only a few hybrids were found. Hybridisation will reduce the number of successful pollinations of C. arenaria, and hence may reduce the reproductive success of C. arenaria over time.
The question that emerges is why is the hybridisation so frequent. Hybridisation in Caladenia is very well known and numerous hybrid combinations have been reported (eg. Backhouse and Jeanes 1995, Bates and Weber 1990). Large hybrid swarms (where F1 & F2 hybrids plus backcrosses to the parents are present), as observed in C. arenaria, are rare (Carr, pers. obs.). Orchid species are generally pollinator specific. Each orchid species is believed to secrete a pheromone analogue of a different wasp species (Bower, 1996), so hybridisation is generally not presumed to occur.
An alternative hypothesis is that C. arenaria does not secrete an analogue of a pheromone, but that it secretes a floral scent indicative of a nectar source. The insect is attracted to the flower, and in the process of searching for the non-existent nectar effects pollination (Col Bower pers. comm.) Under this scenario a greater level of hybridisation would be expected than in a pollination mechanism dependent on sexual deceit.
Feral bees have been suggested as the vector responsible for the frequent hybridisation but there is no evidence to support this in C. arenaria and feral bees have never been reported as Caladenia pollinators.
The Urana population grows adjacent to a paddock used for cropping. There is a potential risk to the population from herbicide or pesticide spray drift. Fertiliser runoff or spray drift may pose a risk to the population by directly inhibiting the orchid, the pollinator, the mycorrhizal partner, or by favouring the weed flora.
The populations on public land are potentially at risk from disturbances resulting from logging practices and the management of travelling stock. A disused gravel pit (for gravel used in roadworks) is adjacent to one of the sites and plants may have been destroyed. Three populations occur in production sections of State Forest. Known populations are excluded from logging. Logging practices may be a threat to any "undiscovered" populations via damage from falling trees, snigging and harvesting machinery (direct damage) or promotion of the weed flora following soil disturbance or opening of the canopy.
Most C. arenaria in State Forests are growing among regrowth White Cypress Pine. This may be due to reduced grazing pressure, suppression of weeds under the regrowth or an interaction between these and other influences. Silvicultural practice is to thin regrowth to reduce competition and promote more rapid growth in the cypress pine stand. Thinning of young White Cypress Pine is likely to be detrimental to populations of C. arenaria, again either by direct physical damage or by promoting the weed flora.
Illegal collection of plants or flowers by orchid enthusiasts or scientists poses some risk to the population. Several holes, probably dug by collectors, were found in 2000 at one population in State Forest. The impact of this single event on the population is minor. Caladenias require skilful management in cultivation and most growers recognise the effort required for successful cultivation and are not interested in collecting. Of greater concern perhaps is the collection of flowers, or removal of plants by people unaware of the cultivation difficulties. Given the fairly remote locations of the populations neither of these scenarios is likely to pose a serious long-term threat to the species .
Recovery may require some change in management practices on State Forest and possibly the travelling stock reserve, but these are not likely to be significant due to the small area involved. Management of the orchid population on the freehold site at Urana could potentially be improved by avoiding grazing during the growing period (July - November). The cost of this measure could be minimised by fencing the perimeter of the population (about 12 ha) so that management practices in the remainder of the paddock are unchanged. Access for stock within the fenced area would be required (outside the growing season) to manage the grass sward. Otherwise the orchids could become competitively disadvantaged by rank grass growth.
The social consequences of failing to implement a recovery program are high. It is likely that C. arenaria has become extinct at two sites in the last ten years. All of the currently known sites possess threats impinging on C. arenaria, with one of these sites only possessing 20 plants. Without appropriate action, this species is likely to rapidly decline into extinction.
At all locations where C. arenaria occurs it is accompanied by other rare, vulnerable or endangered orchid species (Table 1), many of which are undescribed and seven of which are apparently endemic in the Riverina region of NSW. Some of the taxa are rarer and/or more restricted than C. arenaria. It is probable that further study will reveal other rare orchid taxa at these locations. In addition, depleted, rare or vulnerable or endangered plant species other than orchids are likely to occur at C. arenaria locations, especially the Urana site which carries the best extant example of a very rare, endangered vegetation type (Benson et al. 1996).
Many of the 13 orchid taxa listed in Table 1 are eligible for listing under the TSC Act but have not yet been nominated. For many, especially Caladenia species, management requirements will be very similar or identical to the management actions advocated for C. arenaria in this Recovery Plan (although these aspects require specific study). There are considerable, clearly-identifiable biodiversity benefits from management of the C. arenaria sites at the vegetation community level, as vegetation in the Riverina has been severely depleted and degraded, and at the level of plant taxa, especially for orchids. Many of the actions advocated here for C. arenaria (eg. weed control, prescribed burning) will favour the orchid flora generally; none of the management actions are considered antagonistic to other orchid taxa.
|Taxon||Conservation status+Australia (upper case) and NSW (lower case)||Australian distribution||Caladenia arenaria location / population||References|
|TSR||Buckingbong SF||Lonesome Pine SF||Urana|
|Caladenia callitrophila D.L. Jones||E, e||NSW Riverina endemic||+||Jones (1999), Bishop (1996) (p.150), G. Carr (unpubl. data)|
|Caladenia deformis R.Br.||R||WA, SA, Vic, NSW, Tas||+||Bishop (1996), J. Riley (pers. comm.)|
|Caladenia flaccida D.L. Jones||R, r||Vic, NSW, Qld, SA||+||Jones (1991), Bishop (1996), J. Riley (pers. comm.)|
|Caladenia rileyi D.L.Jones||V, v||NSW Riverina endemic||+||Jones (1997)|
|Caladenia stellata D.L.Jones||R, r||NSW, SA||+||Jones (1991), Bishop (1996), J. Riley (pers. comm.)|
|Caladenia sp. nov. aff tentaculata Schltdl. (Riverina)||R, r||NSW Riverina endemic||+||+||G. Carr (unpubl. data), J. Riley (pers. comm.)|
|Caladenia sp.nov. (Urana)||E, e||NSW Riverina endemic||+||G. Carr (unpubl. data)|
|Caladenia xanthochila D. & C. Beardsell||E, e||Vic, NSW||+||Beardsell & Beardsell (1992), Bishop (1996), G. Carr (unpubl. Data)|
|Diuris sp. nov. aff. behrii Schtldl. (Riverina)||E, e||NSW Riverina endemic||+||D. Jones (pers. comm.), J. Riley (pers. comm.)|
|Diuris sp. nov. aff. maculata Sm. (Riverina)||E, e||NSW Riverina endemic||+||J. Riley (pers. comm.), G. Carr (unpubl. Data)|
|Diuris sheaffiana Fitzg.||R, r||NSW, Qld, Vic||+||+||+||-|
|Diuris sp. "Oaklands"||E, e||NSW Riverina endemic||+||G. Robertson (unpubl. Data)|
|Prasophyllum cf. campestre R. J. Bates & D. L. Jones||R, r (?)||NSW, Qld||+||+||D. Jones (pers. comm.)|
|Prasophyllum sp. nov. aff. odoratum R. S. Rogers||V, v (?)||NSW Riverina endemic||+||D. Jones (pers. comm.)|
+ Conservation status based on literature (see references), personal communication (as cited in References) or opinion of Recovery Plan authors.